You know a pain in the butt about the multi-language stuff now is that
there is no fallback to the default language, if an item is not
translated yet. Now I know this might not always be wanted, but is sure
is needed if someone were to add another language to a running site and
translation would take a while.
So my suggestion would be to add a flag to a content-item to specify it
as being a fall-back item for other languages - and it being checked by
default. And another global swith defining wether fallback is enabled or